this is a very rough non technical descritpion
a wire is run from the existing pstn port to the dslam in port, then from the dslam out port to the line connection.
llu etc etc where the non bt cp provides dial tone also.
pstn and adsl combined are spat out the llu equipment, a wire is them run to test matrix in, then from test matrix out to line connection.
these comments are my own and in no way represent any company that i may or may not be linked too.